ENH: Check whether `eg` and `ehe` groups consist of max 2 individuals

#### Is your feature request related to a problem?

As far as I can see, there are no checks whether groups created by `eg_id_numpy` and `ehe_id_numpy` are at most of size 2.

#### Describe the solution you would like to see

GETTSIM should throw an error if the user-provided inputs imply larger groups.